NVIDIA HB SLI Bridge: A Technical Review

Along with the latest GeForce GTX 1080 graphics card, we were introduced to the new NVIDIA HB SLI Bridge. In this article we have a closer look at the internals of the new SLI bridge.

NVIDIA HB SLI Bridge Introduction

In the past, graphics cards powered by an NVIDIA GPU sporting one Scalable Link Interface (“SLI”) connector supported only 2-Way SLI. Graphics cards with two SLI connectors supported 3-Way and 4-Way SLI. The majority of SLI systems use two graphics card in 2-Way SLI configuration, so NVIDIA decided to combine both connectors to improve the communication between the graphics card’s GPUs by increasing the bandwidth. The new bridge is called High Bandwidth (“HB”) bridge.

 

geforce-gtx-titanx-back

source: legitreviews.com

The SLI connector itself is mainly for communication between the master and the slave card(s). There are several different SLI modes such as AFR or SFR and the cards have to communicate with each other to coordinate the frame rendering. NVIDIA has brief documentation available on the various SLI configuration methods on their website, however there is little public information available on the specific data that is transferred over the bridge. Considering that NVIDIA has supported SLI communication over the PCIe interface, allowing SLI without the need for a bridge, it’s not unreasonable to assume SLI doesn’t require a high bandwidth bridge to operate.

Apart from the performance scaling, micro-stuttering is one of the biggest problems of SLI. According to NVIDIA the increased bandwidth of the new SLI bridge should also help to improve the SLI experience by reducing the micro-stuttering.

Legacy SLI bridges operate at a frequency of 400 MHz and only use one SLI finger (connector). The NVIDIA HB SLI Bridge can operate at 650 MHz and use both SLI fingers which should increase the bandwidth in theory and also allow a faster communication between both cards.

nvidia-geforce-gtx-1080-recommended-sli-bridge-configuration

NVIDIA recommends to use either the LED or HB SLI Bridge for any operation above 1080p resolution. The LED bridges launched back 2014, which begs the question: what’s all the fuss about? Apart from using both SLI connections, what makes the HB Bridge special?

 

Nvidia HB SLI Bridge

Nvidia HB SLI Bridge

Reading reviews on several other websites you can see that there is either a) no difference at all, b) a very small difference or c) suddenly a huge performance increase in some specific applications or games.

So I decided to do my own performance testing. Along with studying the effect on performance, I also wanted to find out more about the electrical and physical differences between the various SLI bridges. I sent a couple of different SLI bridges to a special lab for X-ray analysis. The X-ray analysis shows traces inside the PCB which you usually can’t see and also indicates the quality of the solder connections.

(PSA: X-ray analysis is not cheap. If you feel my content is worthwhile and you want to support me, check out the affiliate links below)

 

Available NVIDIA SLI Bridges

Below a list of available NVIDIA SLI bridges
VendorProductTypeSLI SupportSpacingAmazon
ASUSRepublic of Gamers Flexible 2-Way SLI BridgeFlexible2-Way40mmLink
ASUSSLI Flexible BridgeFlexible2-Way70mmLink
ASUSSLI Flexible BridgeFlexible2-Way120mmLink
ASUS3-Way SLI Flexible BridgeFlexible3-WayLink
TekitSLI Flexible BridgeFlexible2-Way150mmLink
EVGA PRO SLI Bridge HBHB2-Way40mmLink
EVGA PRO SLI Bridge HBHB2-Way120mmLink
NVIDIA GeForce GTX SLI HB Bridge 3-Slot HB2-Way60mmLink
NVIDIAHB SLI Bridge 4-slotHB2-Way120mmLink
ASUS ROG Enthusiast 2-Way SLI Bridge Solid2-Way40mmLink
ASUS ROG Enthusiast 3-Way SLI Bridge Solid3-WayLink
ASUSNvidia 3-Way SLI BridgeSolid3-WayLink
ASUS ROG Enthusiast 4-way SLI Bridge Solid4-WayLink
EVGA Pro SLI Bridge V2Solid2-Way40mmLink
EVGA Pro SLI Bridge V2Solid2-Way60mmLink
EVGA Pro SLI Bridge V2Solid3-WayLink
EVGA Pro SLI Bridge V2Solid4-WayLink
GIGABYTE3-Way SLI BridgeSolid3-WayLink
MSI 2-WAY SLI BRIDGE L TITANIUMSolid2-Way40mmLink
MSI Gaming 2-WAY SLI BRIDGE LSolid2-Way40mmLink
NETCNA 3-Way SLI BridgeSolid3-WayLink
NVIDIA2 Way Single SLI BridgeSolid2-Way40mmLink
NVIDIA2-Way Spaced BridgeSolid2-Way60mmLink

 

SLI Bridge X-Ray Analysis

Flexible 2-Way Bridge

Below you see pictures of the X-ray analysis of a flexible 2-way SLI bridge from ASUS. It’s a very common SLI bridge which I picked out of the box of my Rampage V Edition 10 (Amazon). A flexible bridge is variable in length and thus very useful for all kind of SLI configurations.

NVIDIA_FLEX_SLI_Stecker_r NVIDIA_FLEX_SLI_Stecker_r_02 - Kopie NVIDIA_FLEX_SLI_Übersicht_01 - Kopie

NVIDIA_FLEX_SLI_Übersicht_01 NVIDIA_FLEX_SLI_Übersicht_02

You can clearly see that the SLI bridge is just a plain point-to-point connection. There are no special components such as ICs, capacitors or resistors. You can also see voids inside the soldering (white spots inside the dark soldering area), but the amount is still within tolerance and won’t cause any problems or signal loss.

 

Solid 2-Way Bridge

The next bridge is solid comes from ASRock and is also meant for 2-way SLI. Looking at the X-Ray pictures you can also see a lot of traces and also vias which connect different PCB layers. In this case the top and bottom layer are ground (GND) and connected with the vias. The solder quality is a little bit worse than the flexible ASUS bridge but still fine to use. Technically there is no difference compared to the flexible bridge – also here no special components.

NVIDIA_3S_Card_SLI_Stecker_r NVIDIA_3S_Card_SLI_Übersicht_schräg

NVIDIA_3S_Card_SLI_Übersicht_01 NVIDIA_3S_Card_SLI_Übersicht_02

 

Solid 3-Way Bridge

The ASUS 3-Way SLI bridge is technically also very similar to the bridges above. Solder quality is decent and you can mainly see straight connections between the different connectors. You can see that this is clearly a multi-layer bridge with 4 PCB layers (notice: different gray shades which indicate the X-ray depth).

ASUS_3-WAY_SLI_stecker_ul ASUS_3-WAY_SLI_stecker_ur ASUS_3-WAY_SLI_Übersicht_01 ASUS_3-WAY_SLI_Übersicht_03 ASUS_3-WAY_SLI_Übersicht_schräg

 

NVIDIA 2-Way LED Bridge

So let’s go over to the more interesting part: NVIDIA’s LED Bridge. Just by looking at the first picture you will notice that there is a slight difference compared to the other bridges. Some traces are not straight but wave-shaped. This bridge has an adjusted trace-length to make sure all traces on the bridge have exactly the same length. This will improve the signal quality for high frequency operations, which makes sense if you increase the frequency from 400 to 650 MHz.

NVIDIA_HF_SLI_Stecker_l NVIDIA_HF_SLI_Übersicht_01 - Kopie NVIDIA_HF_SLI_Übersicht_02 NVIDIA_HF_SLI_Übersicht_schräg

The parts which you can see in the middle are additional ICs, resistors and capacitors which are just for the LED operation. None of these components have influence on function or performance.

NVIDIA_HF_SLI_sonstige_Bauteile

In addition the solder quality is a bit better than compared to the other 3 bridges listed above. There is only a very small amount of voids inside the soldering.

 

EVGA 2-Way HB SLI Bridge

The EVGA HB Bridge is essentially the same as two normal NVIDIA LED bridges. The HB Bridge is also optimized for the best signal quality with adjusted traces and also with good solder quality.

NVIDIA_EVGA_SLI_Stecker_Übersicht_schräg

NVIDIA_EVGA_SLI_sonstige_Bauteile NVIDIA_EVGA_SLI_sonstige_Bauteile_02 NVIDIA_EVGA_SLI_Stecker_or

NVIDIA_EVGA_SLI_Übersicht_01 NVIDIA_EVGA_SLI_Übersicht_02

The additional components on this bridge are also just for LED control and not related to the SLI performance.

 

SLI Bridge Electrical Analysis

From a technical point of view the new SLI bridge is not quantum leap in either electrical engineering or design. Essentially it’s the same as two “LED bridges” combined in one PCB. For sure the signal quality is better than using a old flex bridge. However it seems a bit over the top that NVIDIA claims a huge effort in development while adjusting traces to the same length is usually just one click in the development software and takes only few seconds.

Apart from the adjusted trace length all bridges look very similar and I couldn’t find any major difference. So the question is: How does the NVIDIA driver identify the HB Bridge if there is no technical difference? So I grabbed my multi-meter and tracked down the traces on the SLI bridge to see what they actually do.

Each SLI connector has 26 pins in total. However, if you count the traces on the PCB you will only find 20 traces.

I found out that 20 pins are for data transfer and 6 pins are connected to ground (GND) on the flexible legacy bridge.

WP_20160810_15_15_35_Pro

Legacy SLI bridge pinout

 

The LED / HB Bridge is slightly different. One pin is not routed to ground but no supplies 1.8 Volt to the SLI Bridge. This supply voltage is also used to feed the LED. In addition the driver can notice if the pin is bridge to ground or not. Bridged to ground, the driver detects a legacy bridge. Not bridged to ground it will detect a HB bridge.

 

WP_20160809_10_06_48_Pro

EVGA HB Bridge Pinout

 

Knowing that there is no difference apart from the tracing I didn’t really expect too much. Even though I already read other reviews online I still wanted to test the performance myself.

 

NVIDIA SLI Bridge Performance Test

I tested the new HB Bridge vs 1 LED Bridge vs 1 and 2 flexible legacy bridges. I know you don’t love synthetic benchmarks such as Fire Strike so I also tested Crysis 3, Far Cry Primal and Rainbow Six Siege. I ran all games with 4K resolution and the highest settings available.

 

Test Setup

 

NVIDIA SLI Bridge Test Results

1 2 3 4

The results are rather disillusioning. 3DMark Fire Strike Extreme benefits a bit and gains about 200 points going from legacy to a new bridge. Crysis 3 doesn’t scale at all, Far Cry Primal about 1 FPS and Rainbow Six Siege gains about 3-4 FPS which is an increase of roughly 5 %. Overall I could see no difference between the LED bridge and the new HB bridge.

 

NVIDIA HB SLI Bridge: Conclusion

Yay or nay? Neither of both I’d say. The NVIDIA HB SLI Bridge is slightly better than the legacy bridges but the difference is very small. It seems like you can gain few extra FPS in some games and it’s also likely that the new bridge will improve the gaming experience overall by reducing the micro-stuttering. Is the bridge worth USD $45? I’d say no, but taking into account how much you have to pay for two GeForce GTX 1080 graphics cards, chucking out another 45 dollar isn’t going to make the difference.

If you plan to get a multi-GPU system you might as well get the new High Bandwidth SLI bridge, but don’t expect a huge performance difference. If you already own a LED bridge, there is no point in upgrading apart from bragging rights.

 

Are you using the High Bandwidth bridges? Let me know your thoughts on this topic below. Thanks for reading and until the next one!

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10 comments

  • Dragonmike

    Very nice analysis of the hardware part, der8auer, as usual! Enjoyed your videos accordingly.

    Your should however perform more game tests, as the small selection you chose did not yield any significant results. If you watch this video from the guy at Hardware Unboxed, you’ll start seeing quite substantial differences in games like Witcher 3, The Division and Fallout 4: https://www.youtube.com/watch?v=mWcsaociTjE

    Furthermore (if you jump to approx 8m40s into the video), it seems that two flex bridges give the exact same boost as the HB bridge on these 3 games. The guy doesn’t actually know why that is, that’s where I’m sure you could make the difference 😉

  • K404

    I LOVE this article. An important question answered. Thankyou very much for taking the time (and expense) to deal with it properly.

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  • Johnny

    isn’t the true purpose of the High Brandwidth bridge is help the Frame Time/Frame Latency not so much as the Frame Rate?

    Should re-do this test with Frame Time/Frame Latency

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  • DAn

    …So, what I’m reading here is that if the driver picks up that it’s a HB bridge, it will run at 650mhz but if it is legacy, 400mhz… Does this mean it would be possible to modify a legacy cable (remove 5 GND wires) to trick the bridge into running at 650mhz?

    You say 45USD which is a bit of money, especially if you need a 3 or 4 slot it can be two or three times that amount… Combine that with only SLI 1070’s and it’s no longer worthwhile with cost vs performance, but it is extra performance you could have if there was an alternative…

    Thoughts?

    • Yes you can (at least with my PCB Tri-SLI bridge from AsRock)!

      I tried by using sticky tape on the relevant GND pins of my 1080 GPUs before possibly destroying my SLI bridge by modifying it.
      The performance gains in 3D Mark were immediate but I also had bad graphics artefacts then. But this seems to be due to inaccurate sticking work. So I removed the tapes again from my primary GPU and left the ones on the 2nd GPU and now it works flawlessly.

      NVIDIA control panel no longe shows “… higher performance sli bridge” and performance gains in 3D Mark (points) are about 12% in Fire Strike Ultra (4K) and 7% in normal Fire Strike if I remember right. Also, the overall rendering in games seems more smooth and input lag due to V-Sync minimized.
      In 3D Mark 11 on the other side I couldn’t notice any performance gains. So the HB bridge seems to mainly give an advantage when having high quality and / or 4K rendering.

      Regards

  • Stampede

    Nice write-up. I have been looking around the net for information on the HB bridges.

    Hardware unboxed showed some good gains, but I feel that the HB bridge helps when you using 8x 8x pcie lanes and not so much when you use 16×8 or 16×16.

    It would be nice if this could be tested as well.

  • Hi!

    I tried using sticky tape on the relevant GND pins of my 1080 GPUs before possibly destroying my SLI bridge by modifying it.
    The performance gains in 3D Mark were immediate but I also had bad graphics artefacts then. But this seems to be due to inaccurate sticking work. So I removed the tapes again from my primary GPU and left the ones on the 2nd GPU and now it works flawlessly.

    NVIDIA control panel no longe shows “… higher performance SLI bridge” and performance gains in 3D Mark (points) are about 12% in Fire Strike Ultra (4K) and 7% in normal Fire Strike if I remember right. Also, the overall rendering in games seems more smooth and input lag due to V-Sync minimized.
    In 3D Mark 11 on the other side I couldn’t notice any performance gains. So the HB bridge seems to mainly give an advantage when having high quality and / or 4K rendering.

    Regards

  • P.S. Thank you for your writing and your investigations!
    This gave me the crucial input and helped me save some money since I didn’t really know how much the gains will be…

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